CN104245999B - High-strength hot-dip zinc-coated steel sheet and manufacture method thereof - Google Patents

Abstract

A kind of hot-dip galvanizing sheet steel, wherein, the coating adhesion amount on surface of steel plate with every one side is 20～120g/m²Zinc coating, in this coating, carbide exists for the ratio of more than 5 and less than 50 in each subregion, its mean diameter is below 10nm, oxide exists for the ratio of more than 5 and less than 50 in each subregion, its mean diameter is more than 50nm, described steel plate has following one-tenth and is grouped into: in quality %, containing more than C:0.02% and less than 0.30%, more than Si:0.01% and less than 2.5%, more than Mn:0.1% and less than 3.0%, more than P:0.003% and less than 0.08%, below S:0.01%, more than Al:0.001% and less than 0.20%, more than Ti:0.03% and less than 0.40%, and surplus is made up of Fe and inevitable impurity, wherein, described each subregion refers to by thickness of coating (t₁μm) and by coating cross section and the orthogonal direction of thickness direction on the area (t that obtains so that 1 μm of interval carries out splitting₁×1(μm²))。

Description

High-strength hot-dip galvanized steel sheet and manufacturing method thereof

technical field

The invention relates to an alloyed hot-dip galvanized steel sheet suitable as an antirust surface-treated steel sheet for automobiles and a manufacturing method thereof.

Background technique

TS440MPa or lower hot-rolled steel sheets have been used in the frame and running system of automobiles and trucks. However, recently, in order to improve the crash resistance of automobiles and protect the global environment, high-strength and thin-walled steel sheets for automobiles are being promoted, and studies have begun to use high-strength hot-rolled steel sheets of TS590MPa, TS780MPa, and TS980MPa or higher. .

Automobile components are mostly complex-shaped components obtained by press molding, and materials with high strength and excellent processability are required. On the other hand, from the viewpoint of ensuring the antirust performance of the vehicle body while reducing the thickness of the steel sheet, surface-treated steel sheets with antirust properties imparted to raw steel sheets are desired. Among them, corrosion resistance after painting, welding Alloyed galvanized steel sheet that is excellent in durability and can be manufactured at low cost.

Conventionally, several high-strength hot-rolled steel sheets or hot-dip galvanized high-strength steel sheets excellent in workability and methods for producing them have been proposed. For example, Patent Document 1 discloses a high-strength steel sheet having a tensile strength of 590 MPa or more and an excellent workability and a method for producing the same, which are characterized in that C: 0.02 to 0.06%, Si≤0.3 %, Mn: 0.5-2.0%, P≤0.06%, S≤0.005%, Al≤0.06%, N≤0.006%, Mo: 0.05-0.5%, Ti: 0.03-0.14%, and the balance is substantially composed of Fe The steel is smelted and hot-rolled under the condition that the finish rolling temperature is 880°C or higher and the coiling temperature is 570°C or higher, so that it is substantially ferrite single-phase structure, and the average particle size is dispersed and precipitated. Carbides containing Ti and Mo less than 10nm.

In addition, Patent Document 2 discloses a method of manufacturing a hot-dip galvanized high-strength hot-rolled steel sheet, which is characterized in that C: 0.01 to 0.1%, Si≤0.3%, Mn: 0.2 to 2.0%, P≤0.04%, S≤0.02%, Al≤0.1%, N≤0.006%, Ti: 0.03～0.2%, containing more than one of Mo≤0.5% and W≤1.0%, the balance is Fe The steel composed of unavoidable impurities is smelted, hot-rolled in the austenite single-phase region, and coiled at a temperature above 550°C to produce a ferrite single-phase hot-rolled steel sheet, and then further remove the oxide scale, in this Hot-dip galvanizing is carried out under the state, thus, satisfying 4.8C+4.2Si+0.4Mn+2Ti≤2.5 in terms of mass %, the structure is ferrite with an area ratio of 98% or more, and dispersed in the following Precipitates smaller than 10 nm of Ti and one or more of Mo and W are contained within the range satisfying (Mo+W)/(Ti+Mo+W)≧0.2 in terms of atomic ratio.

However, in Patent Documents 1 and 2, in order to precipitate fine carbides containing Ti, Mo, etc. in ferrite, a coiling temperature (hereinafter also referred to as CT) of 550° C. or higher is required after finishing rolling. Roll down. When coiling a hot-rolled base material containing Si, Mn, and other elements that are more easily oxidized than Fe (hereinafter also referred to as easily oxidizable elements) under such high CT conditions, the surface layer of the steel plate base The generation of internal oxides containing easily oxidizable elements causes the Zn-Fe alloying reaction to be excessively promoted in the subsequent hot-dip galvanizing treatment and alloying treatment, resulting in a problem that the adhesion of the plating layer deteriorates. In addition, when a large amount of internal oxides are present in the surface layer of the base steel sheet, there is a problem that the internal oxides serve as starting points during stretch flange processing, and microcracks occur in the surface layer of the steel sheet and the coating, and the stretch flanged portion Corrosion resistance deteriorates after painting.

On the other hand, when the coiling treatment is performed by lowering the CT in order to suppress internal oxides generated during hot rolling, the strength and workability decrease due to insufficient precipitation of carbides and growth of structures such as pearlite. Furthermore, when the steel sheet is annealed in the subsequent continuous hot-dip galvanizing facility, hydrogen gas is occluded, causing a problem of degradation of hydrogen embrittlement resistance.

prior art literature

patent documents

Patent Document 1: Japanese Patent Laid-Open No. 2002-322543

Patent Document 2: Japanese Patent Laid-Open No. 2003-321736

Contents of the invention

The problem to be solved by the invention

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a metal coating having excellent coating adhesion at the bent portion and post-painting corrosion resistance of the stretched flange processing portion and excellent hydrogen embrittlement resistance while ensuring good workability. high-strength hot-dip galvanized steel.

method used to solve the problem

The inventors of the present invention have repeatedly studied the plating treatment of high-strength steel sheets, and obtained the following findings as a result.

First, the present inventors found that in order to obtain a plated steel sheet excellent in hydrogen embrittlement resistance, the components in the plated layer, especially the average particle size of oxides and carbides are extremely important. The reason for this is considered to be that, when carbides with an average particle size of 10 nm or less and oxides with an average particle size of 50 nm or more exist in the surface layer of the steel sheet, they function as capture sites for hydrogen intrusion, and by suppressing the diffusivity in the steel sheet, hydrogen concentration, suppressed the sensitivity to delayed fracture. In addition, when subjected to compressive strain during press molding, cracks are generated and propagated in the plating layer. In the present invention, due to the presence of fine carbides and oxides, the fine carbides and oxides have a pinning effect at the crack generation portion. It is presumed that this pinning effect stops the propagation of cracks, prevents large peeling, and improves the adhesion of the plating layer during bending. As a result, the corrosion resistance after painting improves.

Next, regarding its production method, the present inventors found that CT is controlled in order to suppress internal oxides generated during hot rolling, and the heating temperature is specified when the steel sheet is subsequently annealed to carbonize the solid solution Ti existing in the surface layer of the steel sheet. The form of the substance precipitated is important. In addition, the present inventors have found that in order to stably precipitate Ti oxide during annealing, it is necessary to use the oxide layer obtained by oxidizing the surface layer of the steel sheet in the heating zone as a base for oxidizing the inside of Ti during reduction annealing in the soaking zone. Oxygen supply sources are effectively utilized. In addition, the present inventors found that in order to stably precipitate Ti carbides during reduction annealing in the soaking zone, the partial pressure of water vapor in the furnace atmosphere should be controlled and hydrogen partial pressure is extremely important. As a result, these carbides and oxides are sucked into the coating during hot-dip galvanizing and alloying and exist in the coating, thereby improving the corrosion resistance after coating, the adhesion of the coating, and the resistance to hydrogen embrittlement.

The present invention was completed based on the above findings, and its features are as follows.

[1] A hot-dip galvanized steel sheet characterized in that it has a zinc coating with a coating weight of ²⁰ to 120 g/m2 per single surface on the surface of the steel sheet, and in the coating, the average particle size is 10 nm or less The carbides exist in a ratio of 5 to 50 in each partition, and the oxides with an average particle diameter of 50 nm or more exist in a ratio of 5 to 50 in each partition,

The steel sheet has the following composition: by mass %, C: 0.02% to 0.30%, Si: 0.01% to 2.5%, Mn: 0.1% to 3.0%, P: 0.003% to 0.08% or less, S: 0.01% or less, Al: 0.001% or more and 0.20% or less, Ti: 0.03% or more and 0.40% or less, and the balance is composed of Fe and unavoidable impurities,

Wherein, each division mentioned above refers to the area (t ₁ ×1(μm ² )) obtained by dividing the plating layer thickness (t ₁ μm) and the plating layer cross section at 1 μm intervals in the direction perpendicular to the thickness direction.

[2] The high-strength galvanized steel sheet according to [1], wherein the carbide contains Ti, and the oxide contains a compound selected from TiO ₂ , MnO, MnO ₂ , SiO ₂ , Al ₂ O ₃ , One or more oxides among Mn ₂ SiO ₄ and MnSiO ₃ .

[3] The high-strength galvanized steel sheet according to [1] or [2], wherein the composition of the steel sheet further contains Nb: 0.001% to 0.2% and V: One or more of 0.001% to 0.5%, Mo: 0.01% to 0.5%, and W: 0.001% to 0.2%.

[4] The high-strength galvanized steel sheet according to any one of [1] to [3], wherein the composition of the steel sheet further contains B: 0.0005% to 0.005% by mass % the following.

[5] The high-strength galvanized steel sheet according to any one of [1] to [4], wherein the steel sheet is a hot-rolled steel sheet.

[6] A method of manufacturing a high-strength hot-dip galvanized steel sheet, characterized in that the steel having the composition described in any one of [1], [3], and [4] is hot-rolled, and After cooling and coiling, continuous annealing and hot-dip galvanizing,

Set the finish rolling finish temperature at 850°C or higher, set the coiling temperature at 540°C or lower, and perform continuous annealing under the following conditions,

(a) Carry out oxidation treatment as follows: the composition of the combustion gas in the heating zone of the annealing furnace is set to H ₂ ≥ 40% by volume, CH ₄ ≥ 20% by volume, CO ₂ ≥ 1% by volume and the balance is CO, N ₂ , C _x H _y (x≥2, y≥4), when the furnace temperature in the heating zone is above 500°C and below 1000°C, the steel plate is heated to above 520°C and below 650°C, forming An oxide layer with a thickness of 6-60nm,

(b) Next, make the atmosphere in the soaking zone contain 5% by volume or more and 50% by volume or less of hydrogen with the balance being N ₂ , and the water vapor partial pressure and hydrogen partial pressure The following formula (1) is satisfied, and the reduction annealing is performed so that the temperature reached in the soaking zone of the steel sheet is 630° C. or more and 780° C. or less,

in, Indicates the partial pressure of water vapor (Pa), Indicates hydrogen partial pressure (Pa).

[7] The method for producing a high-strength galvanized steel sheet according to [6], wherein after the hot-dip galvanizing treatment, the steel sheet is further heated to a temperature of 450° C. to 510° C. for alloying For the treatment, cooling to 400° C. at a temperature of 20° C./sec or less, so that the Fe content in the plating layer is in the range of 7 to 15%.

In addition, in this invention, high strength means that tensile strength TS is 590 MPa or more. In addition, the galvannealed steel sheet of the present invention includes any of cold-rolled steel sheets and hot-rolled steel sheets, and hot-rolled steel sheets are particularly preferable from the standpoints of stretch-flangeability and hole-expandability.

Invention effect

According to the present invention, it is possible to obtain a high-strength galvanized steel sheet having excellent coating adhesion at bent portions, post-coating corrosion resistance at stretched flanged portions, and excellent hydrogen embrittlement resistance while ensuring good workability.

detailed description

Hereinafter, the present invention will be specifically described. In addition, in the following description, the unit of content of each element of a steel component composition is "mass %", and below, unless otherwise specified, it will only express with "%".

Hereinafter, the present invention will be described in detail.

(1) Composition of the steel plate

C: 0.02% to 0.30%

C is an element required to precipitate carbides in the steel sheet, and 0.02% or more is required for this purpose. On the other hand, if it exceeds 0.30%, the weldability will deteriorate, so the upper limit is made 0.30%.

Si: 0.01% to 2.5%

Si is effective as a solid-solution strengthening element, and needs to be contained in an amount of 0.01% or more in order to express the strengthening effect. On the other hand, if it is contained in a large amount exceeding 2.5%, Si oxides will be concentrated on the surface of the steel sheet during annealing, causing non-plating defects and deterioration of coating adhesion, so the upper limit is set at 2.5%.

Mn: 0.1% to 3.0%

Mn is added to increase the strength, and must be contained in an amount of 0.1% or more in order to express the strengthening effect. On the other hand, if the content exceeds 3.0%, oxides of Mn will be concentrated on the surface of the steel sheet during annealing, causing non-plating defects and deterioration of coating adhesion, so the upper limit is set at 3.0%.

P: 0.003% or more and 0.08% or less

P is one of the elements that are unavoidably contained, and if it is less than 0.003%, the cost may increase, so it is set at 0.003% or more. On the other hand, when P is contained in excess of 0.08%, weldability deteriorates. In addition, the surface quality deteriorates. In addition, if the alloying temperature is not raised during the alloying treatment, the desired degree of alloying cannot be obtained. If the alloying treatment temperature is increased in order to obtain a desired degree of alloying, the ductility deteriorates and the adhesiveness of the alloyed plating film deteriorates. When the added amount of P is too high, the alloying temperature rises excessively. For the above reasons, P is set to 0.08% or less in order to achieve a balance between the desired degree of alloying, good ductility, and adhesiveness of the alloyed plating film.

S: less than 0.01%

S segregates at grain boundaries. Alternatively, when a large amount of MnS is generated, the toughness is lowered. For the above reasons, it is necessary to set it to 0.01% or less. The lower limit of the S content is not particularly limited, and may be at the level of impurities.

Al: 0.001% to 0.20%

Al is added for deoxidizing molten steel. However, when the content is less than 0.001%, this object cannot be achieved. On the other hand, if the content exceeds 0.20%, a large amount of inclusions will be generated, which will cause defects in the steel sheet. For the above reasons, Al is set to be 0.001% or more and 0.20% or less.

Ti: 0.03% to 0.40%

Ti is an element required to precipitate carbides in the steel sheet to increase the strength, and is also an effective element from the viewpoint of cost. However, when the amount added is less than 0.03%, the amount of precipitates required for increasing the strength is insufficient, and when it exceeds 0.40%, the effect is saturated, leading to an increase in cost. For the above reasons, Ti is set to be 0.03% or more and 0.40% or less.

In addition, in order to control strength and workability, the following elements may be added in addition to the above-mentioned elements.

One or more selected from Nb: 0.001% to 0.2%, V: 0.001% to 0.5%, Mo: 0.01% to 0.5%, W: 0.001% to 0.2%

Nb, V, Mo, and W precipitate as composite carbides containing Ti in the steel sheet, and are elements effective for stabilizing the precipitation of fine carbides, and one or two or more of these elements are added. However, if the added amount is less than the predetermined range, the strength-enhancing effect due to precipitation is insufficient, and if it exceeds the predetermined range, the effect is saturated, leading to an increase in cost. Therefore, when contained, Nb is set to 0.001% to 0.2%, V is set to 0.001% to 0.5%, Mo is set to 0.01% to 0.5%, and W is set to 0.001%. Above and below 0.2%.

B: 0.0005% or more and 0.005% or less

B is an element effective for improving hardenability. However, when it is less than 0.0005%, it is difficult to obtain the effect of accelerating quenching. On the other hand, if the added amount exceeds 0.005%, the effect will be saturated and the cost will increase. Therefore, when contained, B is set to 0.0005% or more and 0.005% or less.

The balance is Fe and unavoidable impurities.

(2) Carbide and oxide in the coating

The high-strength galvanized steel sheet of the present invention is characterized in that the average particle size of carbides present in the coating layer is 10 nm or less and the average particle size of oxides is 50 nm or more. When the average particle size of the carbide exceeds 10 nm, the crack propagation suppressing effect is small, which deteriorates the coating adhesion during processing, and the hydrogen capture effect is small, which deteriorates the resistance to hydrogen embrittlement. When the average particle size of the oxide is less than 50 nm, the hydrogen trapping effect is small and the resistance to hydrogen embrittlement is deteriorated. At the same time, the post-painting corrosion resistance of the processed portion deteriorates due to the generation of cracks. The carbides are present in a ratio of 5 or more and 50 or less per partition. When there are less than 5 per division, the hydrogen trapping effect is small and the resistance to hydrogen embrittlement is deteriorated. When the number of partitions exceeds 50, the workability of the plated coating film deteriorates and the adhesion of the plated layer decreases.

Oxide exists in the ratio of 5 or more and 50 or less per partition. When there are less than 5 per division, the hydrogen trapping effect is small and the resistance to hydrogen embrittlement is deteriorated. When the number of partitions exceeds 50, the workability of the plated coating film deteriorates and the adhesion of the plated layer decreases.

It should be noted that each of the above subregions refers to a certain area of the coating cross-section, which is the area obtained by dividing the coating cross-section with 1 μm intervals in the direction perpendicular to the thickness direction through the coating thickness (t ₁ μm) ( t ₁ ×1 (μm ² )).

In the present invention, the carbide preferably contains Ti. In addition, it is effective that the oxide contains Ti and also contains Si, Mn, and Al. Specifically, it is preferably selected from TiO ₂ , MnO, MnO ₂ , SiO ₂ , Al ₂ O ₃ , Mn ₂ SiO ₄ , and MnSiO ₃ . more than one oxide. This is because the strengthening element added to the steel is precipitated in the form of oxides as much as possible to soften the surface layer of the steel sheet immediately below the plating layer, thereby promoting stress relaxation during working.

In addition, the composition of carbides and oxides in the plating layer can be confirmed by the following method. For example, the method of processing the cross-section of a steel plate into a thin sheet so as to include a coating layer using a focused ion beam processing device (FIB), observing with a transmission electron microscope (TEM), and using an energy dispersive X-ray detector (EDX) ) for composition analysis and electron beam analysis. In addition, as a method of measuring the average particle size of carbides and oxides in the plating layer, a method of measuring the maximum diameter and the minimum diameter from photographs in the above-mentioned observation method and averaging them is used.

In addition, the high-strength galvanized steel sheet of the present invention has a zinc-coated layer with a coating weight of 20 to 120 g/m ² per one side on the surface of the steel sheet. If it is less than 20 g/m ² , it will be difficult to ensure the corrosion resistance after painting, and if it exceeds 120 g/m ² , the adhesion of the plating layer will decrease.

In addition, the high-strength galvanized steel sheet of the present invention is preferably a hot-rolled steel sheet for reasons of stretch flangeability and hole expandability.

(3) Manufacturing method of high-strength hot-dip galvanized steel sheet

Next, the method for producing the high-strength galvanized steel sheet of the present invention and the reasons for its limitations will be described.

First, hot rolling conditions will be described.

Finish rolling finish temperature above 850℃

When the finish rolling finish temperature is lower than 850° C., the cumulative amount of strain due to rolling in a non-recrystallized state increases, leading to an increase in rolling load. Therefore, the finish rolling finish temperature is set to 850° C. or higher. There is no particular limitation on the upper limit. In the present invention, it is preferably 1100°C or lower.

Coiling temperature below 540°C

When the coiling temperature exceeds 540°C, internal oxides will be formed due to easily oxidizable elements, and the Zn-Fe alloying reaction will be excessively promoted during the subsequent hot-dip galvanizing and alloying treatments, resulting in the occurrence of alloying Deterioration of appearance due to unevenness, reduction of plating adhesion of bent parts, and deterioration of corrosion resistance after painting of stretched flange parts. In addition, due to the progress of internal oxidation, Ti required to form carbides is consumed. Therefore, carbide-forming elements such as Ti are consumed due to internal oxidation, thereby forming a Ti-deficient layer. Therefore, it is difficult to allow sufficient Ti carbides to exist in the plating layer. Therefore, the coiling temperature is set to 540° C. or lower.

Next, continuous annealing and hot-dip galvanizing will be described.

The composition of the gas in the heating zone of the annealing furnace is H ₂ ≥ 40% by volume, CH ₄ ≥ 20% by volume, CO ₂ ≥ 1% by volume, the balance of CO, N ₂ , C _x H _y (x ≥ 2, _y ≥4)

When H ₂ , CH ₄ , and CO ₂ are low, the surface activation effect after redox is small, and carbides and oxides formed immediately below the coating during reduction annealing are difficult to be sucked into the coating. Therefore, it is difficult to obtain the most important effect of supplying carbides and oxides into the plating layer in the present invention. There is no particular limitation on the upper limit. As for the remaining gas, the same effect can be obtained as long as these gases are mixed even in an extremely small amount. The combustion gas may be formed by mixing hydrogen gas with natural gas, industrial methane, ethane, propane gas, etc., or coke oven gas produced by a so-called water-gas reaction. However, the combustion calories of coke oven gas vary depending on the operation rate of mines and coke ovens that produce coal as a raw material. Therefore, it may be necessary to adjust the composition by adding hydrogen or the like, so coke oven gas cannot always be used as it is.

The furnace temperature in the heating zone is above 500°C and below 1000°C

When the furnace temperature is lower than 500°C, uneven oxidation will occur on the surface of the steel plate and will not be fully oxidized. Therefore, the effect of absorbing carbides and oxides into the coating will not be uniform. When the temperature exceeds 1000° C., the surface of the steel sheet is excessively oxidized, the interface between the plating layer and the steel sheet becomes rough, and the adhesion of the plating layer during processing deteriorates.

The heating temperature of the steel plate in the heating zone is not less than 520°C and not more than 650°C

When the temperature is lower than 520°C, the surface of the steel sheet is not sufficiently oxidized, so the effect of absorbing carbides and oxides into the coating is small. When the temperature exceeds 650° C., excessive oxidation occurs, the interface between the plating layer and the steel sheet becomes rough, and the adhesion of the plating layer during processing deteriorates.

Oxidation treatment to form an oxide layer with a thickness of 6-60nm on the surface of the steel plate

In the present invention, when the thickness is less than 6 nm, the amount of oxidation on the surface of the steel sheet is insufficient, so the effect of absorbing carbides and oxides into the plating layer is small. When the thickness is larger than 60 nm, excessive oxidation occurs, the interface between the plating layer and the steel sheet becomes rough, and the adhesion of the plating layer during processing deteriorates. In addition, the oxide layer in the present invention refers to an oxide layer mainly containing an Fe oxide layer and substantially not containing Ti (meaning an oxide layer having a Ti content of 0.001% or less). When the heating temperature of the steel plate exceeds 650°C, Ti is absorbed into iron oxide in the form of oxide. This Ti oxide remains at the interface during reduction annealing to roughen the interface between the plating layer and the steel sheet, thereby deteriorating the adhesion of the plating layer during processing, which is not preferable.

The soaking zone atmosphere contains 5% by volume or more and 50% by volume or less of hydrogen and the balance is _N2 , and the partial pressure of water vapor and hydrogen partial pressure Satisfy formula (1).

When H ₂ is less than 5% by volume, the surface of the steel sheet is not sufficiently reduced, and therefore oxides remain at the interface, deteriorating the corrosion resistance after coating. When it exceeds 50% by volume, the steel sheet occludes a large amount of hydrogen gas, so that bubbles and the like are generated in the plated film, thereby deteriorating the surface quality. The margin was set to N ₂ . Also, the partial pressure of water vapor Partial pressure with hydrogen The ratio needs to satisfy the following formula (1).

When it is less than 10 ^-3 (0.0010), Ti will be nitrided without forming carbides. on the other hand, When it is larger than 10 ^-1 (0.1000), Ti is consumed by internal oxidation during annealing, and carbide cannot be formed.

The reached temperature of the steel sheet in the soaking zone during reduction annealing is not less than 630°C and not more than 780°C

When the temperature is lower than 630°C, the surface will not be activated, and the effect of introducing carbides and oxides into the coating cannot be obtained, so it is not preferable. When the temperature exceeds 780°C, Ti is selectively consumed by external oxidation, and carbide cannot be formed. The control of H ₂ O composition is implemented by the following method: a bubbling device is installed outside the annealing furnace, a predetermined flow rate of N ₂ gas is passed into a water tank kept at room temperature, mixed with non-humidified N ₂ gas beforehand, and introduced into the furnace. In addition, at this time, it is necessary to flow gas from the lower part of the annealing furnace. This is because H ₂ O has a light specific gravity, so H ₂ O stays in the upper part of the furnace. Here, the lower portion of the furnace body means up to 1/10 of the entire height of the furnace body.

The method of measuring the H ₂ O and H ₂ partial pressures from the dew point is not particularly limited. For example, a predetermined amount of gas is sampled, and its dew point is measured using a dew point measuring device such as a DewCup to obtain the partial pressure of H ₂ O. Likewise, measure the _H2 partial pressure using a commercially available _H2 partial pressure meter. Alternatively, by measuring the pressure in the atmosphere, the partial pressures of H ₂ O and H ₂ can be calculated from the concentration ratio.

In addition, the hot-dip galvanized steel sheet of the present invention may be subjected to alloying treatment after hot-dip galvanizing to obtain an alloyed hot-dip galvanized steel sheet. In this case, the alloying treatment is performed by heating the steel sheet to a temperature of 450° C. to 510° C., and cooling to 400° C. at a rate of 20° C./second or less. The Fe content in the thus obtained plating layer is 7 to 15%. When the Fe content is lower than 7%, not only the uniform surface appearance cannot be obtained due to uneven alloying, but also the soft ζ phase will be formed thickly on the surface of the coating due to insufficient Zn-Fe alloying reaction, resulting in Plating peels off in scales during bending. On the other hand, when it exceeds 15%, the Zn-Fe alloying reaction proceeds excessively, and a brittle Γ phase is formed thickly near the interface between the coating and the steel sheet, thereby deteriorating the adhesion of the coating.

When the alloying temperature is lower than 450° C., the alloying reaction does not proceed sufficiently. When the temperature exceeds 510° C., the Γ phase is formed thickly to degrade the plating adhesion of the processed portion. After alloying, it is cooled to 400°C at a rate of 20°C/sec or less. When the cooling rate is slow, the Γ phase is formed thickly, deteriorating the adhesion of the plating layer.

Example 1

After heating the steel slab with the composition shown in Table 1 at 1250°C, it was hot-rolled under the conditions shown in Table 2, and the black oxide scale was removed by pickling to produce a hot-rolled steel sheet with a thickness of 2.3 mm. .

Next, continuous annealing and hot-dip galvanizing are performed using the CGL production line. In CGL, coke oven gas whose composition has been adjusted to a predetermined composition is combusted in the heating zone to perform oxidation treatment, and then the furnace atmosphere, water vapor partial pressure, and hydrogen partial pressure are controlled under the conditions shown in Table 2 in the soaking zone. Reduction treatment is carried out on the steel plate according to the maximum attainable temperature of the steel plate. In addition, for the control of the dew point in the atmosphere, a piping through which humidified N ₂ gas flows by heating a water tank installed in the N ₂ gas line is separately provided in advance, and H ₂ gas is introduced into the humidified N ₂ The dew point of the atmosphere gas is controlled by mixing it with the gas and introducing it into the furnace. In addition, the control of the H ₂ concentration in the atmosphere is performed by adjusting the amount of H ₂ gas introduced into the N ₂ gas by using a gas valve.

Then, it was immersed in an Al-containing Zn bath having a bath temperature of 460° C., and a hot-dip galvanizing treatment was performed. At this time, the coating weight is adjusted by gas wiping to 45g/m ² (coating thickness t ₁ : 6μm), 70g/m ² (coating thickness t ₁ : 10μm), 140g/m ² (coating thickness t _1:20 μm). For alloyed hot-dip galvanized steel sheets, alloying treatment is performed after hot-dip galvanizing.

For the galvanized steel sheet (GI) and galvannealed steel sheet (GA) obtained by the above method, the appearance (coating surface appearance), workability, coating adhesion of the bent portion, hydrogen embrittlement resistance, and elongation were examined. Corrosion resistance after painting of the flange processing part. The measurement method and evaluation criteria are as follows. In addition, the size and composition of carbides and oxides in the plating layer were measured by observing and analyzing the plated thin film sample after FIB processing using TEM-EDX and EELS. In addition, the surface layer of the steel sheet after heating was identified and analyzed by X-ray diffractometry.

＜Appearance＞

In terms of appearance, it is judged that the appearance is good (○) when there is no defect in the surface appearance of the plating layer such as non-plating and uneven alloying, and it is judged that the appearance is good when there is a defect in the surface appearance of the plating layer such as non-plating and uneven alloying. Bad (×).

＜Processability＞

Cut the JIS No. 5 tensile test piece from the sample along the direction that is 90° relative to the rolling direction, and carry out the tensile test under the condition that the crosshead speed is constant at 10mm/min according to the provisions of JISZ2241, and measure the tensile strength ( For TS (MPa)) and elongation (El (%)), the case of TS×El≧15000 was regarded as good, and the case of TS×El<15000 was regarded as poor.

In addition, a test piece with a hole was prepared by punching the center of a steel plate cut into a 130 mm square with a 10 mmφ punch with a gap of 12.5%, and pushed upward from the direction opposite to the rough side of the punched hole with a 60° conical punch. , Measure the hole diameter d (mm) when the crack penetrates the steel plate, and calculate the hole expansion rate λ according to the following formula.

λ(%)=[(d-10)/10]×100

＜Coating Adhesion of Bending Part＞

Regarding the coating adhesion of the hot-dip galvanized steel sheet without alloying treatment, after the steel sheet was bent to 180°, the tape was peeled off from the outside of the bent portion, and the presence or absence of the coating peeling was visually judged and evaluated as follows.

○: The coating is not peeled off

×: Plating peeled off

In addition, the coating adhesion of the galvannealed steel sheet was evaluated by the pulverization test shown below.

Adhesive tape was attached to the plated steel plate, with the tape-adhesive side as the inner side, and 90° bending restoration was performed with a bending radius of 5 mm, and the peeled tape was analyzed by fluorescent X-rays. The Zn count per unit length at this time was calculated|required and it was set as the plating peeling amount. With regard to the powdering resistance, referring to the following criteria, the case where the amount of plating peeling obtained in the aforementioned powdering test was ranked 1 was evaluated as good (⊚), and the amount of plating peeling obtained in the aforementioned powdering test was The case of rank 2 was evaluated as almost good (◯), and the case of rank 3 in the case where the plating peeling amount obtained in the above-mentioned pulverization test was evaluated as poor (×). ◎, ○ are qualified.

Coating peeling amount: grade

0 or more and less than 3000:1 (good (◎))

More than 3000 and less than 6000: 2 (good (○))

More than 6000: 3 (bad (×))

＜Hydrogen embrittlement resistance＞

Bend a strip-shaped test piece of 150mm×30mm with a bending radius of 5mm, install a water-resistant strain gauge on the surface, immerse it in 0.5mol/L sulfuric ^acid , and use a current density of 0.1mA/cm The test piece was energized to perform electrolysis, hydrogen was introduced into the test piece, and the occurrence of cracks after 2 hours of energization was evaluated according to the following criteria.

Good (○): No cracks occurred

Bad (×): Cracks occurred

＜Corrosion resistance after painting＞

Use a 10mmφ punch to punch out the center of a steel plate cut into a 130mm square with a gap of 12.5% to prepare a test piece with a hole, and use a 60° conical punch to push upward from the opposite direction of the blank side of the punched hole. Reaming processing. At this time, push upward until the hole expansion ratio reaches a value of 80% where cracks are generated. The test pieces processed in this way were subjected to chemical conversion treatment and electrodeposition coating, and were subjected to a 10-day salt spray test based on JIS Z2371 (2000) to evaluate the presence or absence of swelling of the processed portion.

Good (○): No swelling

Bad (×): There is swelling

The results obtained by the above methods are shown in Table 2-1, Table 2-2, Table 3-1, and Table 3-2 together with the production conditions.

Table 2-2

Table 3-2

According to Table 2-1, Table 2-2, Table 3-1, and Table 3-2, it can be seen that the appearance, workability, coating adhesion of the bent portion, hydrogen embrittlement resistance, and stretch flanged portion of the examples of the present invention The corrosion resistance after painting was good (◯). On the other hand, any of the comparative examples not satisfying the scope of the present invention was evaluated low.

Claims (21)

1. A hot-dip galvanized steel sheet characterized by having a coating adhesion amount per one surface of the steel sheet of 20 to 120g/m²The zinc plating layer of (2), wherein carbides are present in a proportion of 5 or more and 50 or less per division and have an average particle diameter of 10nm or less, oxides are present in a proportion of 5 or more and 50 or less per division and have an average particle diameter of 50nm or more,

the steel plate has the following composition: contains, in mass%, C: 0.02% or more and 0.30% or less, Si: 0.01% or more and 2.5% or less, Mn: 0.1% or more and 3.0% or less, P: 0.003% or more and 0.08% or less, S: 0.01% or less, Al: 0.001% or more and 0.20% or less, Ti: 0.03% to 0.40% inclusive, with the balance being Fe and unavoidable impurities,

wherein each partition is defined by the thickness t of the coating₁μ m and an area t obtained by dividing the cross section of the plating layer at intervals of 1 μm in a direction orthogonal to the thickness direction₁×1μm²。

2. The hot-dip galvanized steel sheet according to claim 1, wherein the carbide contains Ti and the oxide contains a material selected from the group consisting of TiO₂、MnO、MnO₂、SiO₂、Al₂O₃、Mn₂SiO₄、MnSiO₃At least one oxide of (1).

3. The hot-dip galvanized steel sheet according to claim 1 or 2, characterized by further comprising, as a component composition of the steel sheet, in mass%: 0.001% or more and 0.2% or less, V: 0.001% or more and 0.5% or less, Mo: 0.01% or more and 0.5% or less, W: 0.001% to 0.2% inclusive.

4. The hot-dip galvanized steel sheet according to claim 1 or 2, characterized by further comprising, as a component composition of the steel sheet, in mass%: 0.0005% or more and 0.005% or less.

5. The hot-dip galvanized steel sheet according to claim 3, characterized by further comprising, as a component composition of the steel sheet, in mass%: 0.0005% or more and 0.005% or less.

6. The hot-dip galvanized steel sheet according to claim 1 or 2, wherein the steel sheet is a hot-rolled steel sheet.

7. The hot-dip galvanized steel sheet according to claim 3, wherein the steel sheet is a hot-rolled steel sheet.

8. The hot-dip galvanized steel sheet according to claim 4, wherein the steel sheet is a hot-rolled steel sheet.

9. The hot-dip galvanized steel sheet according to claim 5, wherein the steel sheet is a hot-rolled steel sheet.

10. The hot-dip galvanized steel sheet according to claim 1 or 2, wherein the zinc plating layer is subjected to alloying treatment.

11. The hot-dip galvanized steel sheet according to claim 3, wherein the zinc plating layer is subjected to alloying treatment.

12. The hot-dip galvanized steel sheet according to claim 4, wherein the zinc plating layer is subjected to alloying treatment.

13. The hot-dip galvanized steel sheet according to claim 5, wherein the zinc plating layer is subjected to alloying treatment.

14. The hot-dip galvanized steel sheet according to claim 6, wherein the zinc plating layer is subjected to alloying treatment.

15. The hot-dip galvanized steel sheet according to any one of claims 7 to 9, wherein the zinc plating layer is subjected to alloying treatment.

16. The hot-dip galvanized steel sheet according to claim 10, wherein the alloyed galvanized layer has an Fe content of 7 to 15%.

17. The hot-dip galvanized steel sheet according to any one of claims 11 to 14, wherein the content of Fe in the alloyed zinc-plated layer is 7 to 15%.

18. The hot-dip galvanized steel sheet according to claim 15, wherein the content of Fe in the alloyed zinc-plated layer is 7 to 15%.

19. A method for producing a high-strength hot-dip galvanized steel sheet, characterized in that, when the steel having the composition according to any one of claims 1, 3, 4 and 5 is hot-rolled, cooled and coiled after the finish rolling, and then subjected to continuous annealing and hot-dip galvanizing,

the finish rolling temperature is set to 850 ℃ or higher, the coiling temperature is set to 540 ℃ or lower, the continuous annealing is performed under the following conditions,

the following oxidation treatment was performed: the composition of the gas in the heating zone of the annealing furnace was set to H₂More than or equal to 40 volume percent of CH₄Not less than 20% by volume of CO₂More than or equal to 1 volume percent and the balance of CO and N₂、C_xH_yHeating the steel plate to a temperature of 520-650 ℃ to form an oxide layer with a thickness of 6-60 nm on the surface of the steel plate, wherein x is more than or equal to 2 and y is more than or equal to 4,

then, the atmosphere in the soaking zone contains 5-50 vol% of hydrogen and the balance is N₂And partial pressure of water vaporAnd hydrogen partial pressureReducing annealing is performed so that the steel sheet has an arrival temperature in the soaking zone of 630 ℃ to 780 ℃ while satisfying the following formula (1),

${10}^{-3}\&le;P_{H_{2}O}/P_{H_2}\&le;{10}^{-1}---\left(1\right)$

wherein,indicates the partial pressure (Pa) of water vapor,indicating the hydrogen partial pressure (Pa).

20. The method for producing a high-strength hot-dip galvanized steel sheet according to claim 19, characterized in that after the hot-dip galvanizing treatment, the steel sheet is further subjected to an alloying treatment by heating to a temperature of 450 ℃ to 510 ℃.

21. The method for producing a high-strength hot-dip galvanized steel sheet according to claim 20, characterized in that the steel sheet is cooled to 400 ℃ at a temperature of 20 ℃/sec or less after the alloying treatment.